Water quality control in fish ponds

ABSTRACT

There is provided a system for water quality control and improvement in intensive fish culture systems. The aim of the water treatment is to reduce the content of inorganic nitrogen and of organic matter. The system comprises a fish pond, a settling pond, a fluidized bed reactor and a trickling filter. The system according to the invention can be an essentially closed one, except for periodic water replenishment. It is suited for ponds with sweet or salt water.

The system of the invention is suited for water quality control inintensive fish culture systems. The treatment system is capable ofreducing inorganic nitrogen and organic matter in closed aquaculturefacilities (without water exchange) to levels not harmful to fish. Thetreatment system is compact and is especially suitable for fish culturein heated, indoor aquaculture facilities.

BACKGROUND OF THE INVENTION

In recent years, aquaculture (cultivation of fish or other aquaticorganisms) is characterized by a tendency towards growing more fish perunit area. Special ponds have been constructed (Zohar and Rappaport,Zohar, G., Rappaport, U. and S. Sarig, 1985. Intensive culture oftilapia in concrete tanks. Bamidgeh 37: 103-112, 1985; van Rijn et al.,van Rijn, J., Stutz, S, Diab, S. and M. Shilo. 1986. Chemical, physicaland biological parameters of superintensive concrete fish ponds.Bamidgeh 38: 35-43. van Rijn, J. and G. Rivera. 1990. Aerobic andanaerobic biofiltration in an aquaculture unit--Nitrite accumulation asa result of nitrification and denitrification. Aquacult. Engineer. 9:217-234. 1986) that enable stocking densities of up to 50 times thestocking densities generally maintained in conventional fishponds. Asopposed to conventional fishponds, waterquality deterioration proceedsrapidly in these intensive fishponds and without man-made interference,fish mortality would be imminent. DE-A-38 27 716 describes a system forwater quality control in intensive fish culture systems for reducinginorganic nitrogen and organic matter concentrations in an open system.Theoretically, two options exist as to maintaining an adequatewaterquality in these intensive fish culture ponds. Either the ponds arecontinuously flushed with clean (unpolluted) water or the pondwater iscontinuously treated in order to reduce the level of pollutants.Unlimited amounts of clean water to flush the ponds is a luxury which isrestricted to a few geographical areas only. Therefore, treatment of thepondwater is the option of choice in most places.

The accumulation of inorganic nitrogen in intensively cultured fishpondsis one of the major limiting factors preventing a furtherintensification. Inorganic nitrogen (especially ammonia and nitrite) istoxic to fish and it accumulates in the pondwater through excretion ofammonia by the fish and by breakdown of organic solids. Most of thetreatment systems used in aquaculture facilities are designed tofacilitate the growth of nitrifying bacteria which oxidize ammonia vianitrite to nitrate. A drawback of the ammonia removal by means ofnitrification is the subsequent increase in nitrate in the culturesystem. Nitrate concentrations of up to 800 mg liter⁻¹ NO₃₋₋ N have beenrecorded in semi-closed aquaculture facilities where nitrification wasemployed as water purification step. High nitrate concentrations oughtto be prevented for several reasons. Firstly, nitrate at highconcentrations has a toxic effect on several fish species. Secondly, thedischarge of nitrate-rich effluent water is prohibited in many countriesdue to environmental and public health considerations. The maximumlevels of nitrate allowed in the effluent water differ from country tocountry and are as low as 11.6 mg liter⁻¹ NO₃₋₋ N in Europe according tothe European Community directive. Thirdly, under certain conditionsnitrate in the fish culture system is converted to nitrite, a compoundextremely toxic to fish.

A treatment system was developed by our group (van Rijn and Rivera,1990) which was aimed a reducing inorganic nitrogen from the pond waterby means of induction of two microbial process: nitrification (oxidationof ammonia to nitrate) and denitrification (reduction of nitrate to N₂gas). Nitrification was induced in a socalled trickling filtercontaining material previously not used for this purpose.Denitrification was induced in fluidized bed reactors. Asdenitrification is a heterotrophic process (denitrifying bacteriarequire organic matter for growth and metabolism), organic matterderived from the fishpond was led through the fluidized bed reactor.This treatment system was not entirely satisfactory as denitrifyingactivity in the fluidized bed reactors was highly unpredictable andlarge daily fluctuations in nitrate removal were observed. Furthermore,nitrite accumulation by the fluidized bed reactor was found under allrunning conditions tested (van Rijn and Rivera, 1990).

SUMMARY OF THE INVENTION

According to the invention the treatment system has been modified byincorporating a settling stage in which organic matter derived from thefishpond is partially degraded. The supernatant of the settling pond,rich in organic decomposition products, is then used to "fuel"denitrifying activity in the fluidized bed reactor.

This modified treatment system has been demonstrated to be effectiveboth at lab-scale and under experimental field conditions. In one of thestations for fishculture research we managed to maintain fish at adensity of 20 kg m⁻³ in a 50 m³ pond over a three months period(July-September). The system was operated in a virtually, closed fashion(only 3 m³ of water was daily added to the pond in order to compensatefor evaporation losses). Both inorganic nitrogen and organic matter werereduced to levels securing optimal fish growth over the experimentalperiod.

Thus the present invention relates to a closed system for water qualitycontrol in intensive fish culture systems, for preventing accumulationof inorganic nitrogen and organic matter, comprising in combination afish pond, means for passing water to be treated from said pond to asettling pond where organic matter is reduced to CO₂ and mainlyshort-chain volatile fatty acids, a fluidized bed reactor into whichwater from settling pond is pumped and where nitrate is reduced togaseous N₂ and short chain volatile fatty acids are oxidized to CO₂ by aplurality of anaerobic bacteria (anaerobic stage), while water from thepond is pumped through a trickling filter (aerobic stage) where ammoniais oxidized by nitrifying bacteria to nitrate, and returned to the pond.

The invention is illustrated by way of example only with reference tothe enclosed schematical FIG. 1, which is a scheme of a fish pond andwater treatment system of the invention.

As shown in FIG. 1 the fish pond "a" is connected with trickling filter"b", and with settling tank "c", from which water is pumped to the lowerpart of fluidized bed column "d", and after passage through "d" it isreturned to pond "a".

The trickling filter "b" is provided with a large internal surface area,and means are provided in introduce water at its top, preferably byspraying. This water trickles down the filter via a plurality ofchannels. Water is pumped, preferably from the center of pond "a" to thesetting tank (pond) "c", and supernatant water from this pond is pumpedinto the bottom of the fluidized bed column "d", and the water havingpassed this column is fed back to the fishpond "a". The settling tankserves to concentrate the organic material from the fish pond, anddecomposition of this material takes place in this settling tank,resulting amongst others in the formation of acetate. The supernatantfrom this tank is fed to the fluidized bed reactor (column), where theacetate is used as carbon source by bacteria which reduce nitrate togaseous nitrogen. While retention time in the settling tank is of theorder of hours, it is of the order of minutes in the fluidized bedreactor. Generally a settling tank of from about 3 to 10 m₃ is adequatefor use with a fish pond of about 50 m₃, by the way of example, which isillustrative only, a satisfactory experimental system is described inthe following.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated with reference to the enclosed schematicalFigures, in which:

FIG. 1 is a scheme of a fish pond and treatment system;

FIG. 2 illustrates ammonia (A), nitrite (B) and nitrate (C)concentrations in a system of the invention;

FIG. 3 illustrates the same in a control pond.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following parameters are intended to give a rough indication of theorder of magnitude of the operation of a system of the invention:

Generally the volume of the settling tank is of the order of from about3 percent to about 10 percent of that of the volume of the fish pond.

Generally there is passed water from the pond through the tricklingfilter at a rate of the order of from about 20 to about 40 percent ofthe volume of the pond per hour. For a pond of about 50 m³, a filter, atrickling filter of 2 m×2 m ×1.5 n as any corresponding configuration,with about 150to about 300 m internal surface area gave good results.For the same pond, a fluidized bed column of from about 3 to 6 meterheight and 15 to about 25 cm internal diameter can be used, with aretention time of from about 5 to about 15 minutes.

These are data intended only to indicate an order of magnitude, and itis clear that these will vary from pond to pond according to itsspecific conditions, concentration of ammonia etc.

A scheme of the fishpond and treatment system is shown in FIG. 1. Thetrickling filter (b) with dimensions of 2.0×2.0×1.5 m was constructed ofPVC material (Jerushalmi Ltd, Nes Ziona, Israel) in such a configurationthat water sprayed on top of the filter material trickles down thefilter through a large number of smooth channels. The surface areaprovided by this PVC material is 200 m² m⁻³. Water is pumped from thesurface of the fishpond (a) onto the trickling filter at a flow rate of15 m³ h⁻¹. From the center of the fish pond water, rich in particulateorganic matter (see van Rijn and Rivera, 1990), water is pumped to asettling tank (c) with a total volume of 3 m³. Supernatant water fromthe settling tank is pumped into the bottom of a fluidized bed column(d) at a flow rate of 0.6-1.2 m³ ⁻¹. The fluidized bed column has aheight of 4.8 m and an inner diameter of 19.3 cm (net volume: 131.5liters). Retention time of the column varied, therefore, from 6.5-13min. The fluidized bed column was filled with sand (diameter: 0.3-0.9mm) up to 40-50 cm of column height. Water leaving the column through aside-arm at the top is led back into the fishpond. The water flow ratefrom the fishpond (50 m³) to the setting tank is identical to the flowrate from the settling tank to the fluidized bed column. Therefore, theretention time of the settling tank varied from 2.5 to 5 hrs.

Performance

Tile treatment system was tested during the summer of 1992 at anexperimental station in Israel. Common carp (Cyprinus carpio) werecultured in the fishpond (50 m³) at a density of 20±4 kg m⁻³ over aperiod of three months. During this period the treatment system wasoperated with a flow rate of 15 m³ h⁻¹ through the trickling filter anda flow rate of 0.6 m³ h⁻¹ through the settling tank and fluidized bedreactor. Clean water was supplied to the fish pond at a rate of 3 m³day⁻¹ in order to compensate for evaporation losses.

Inorganic nitrogen removal

Concentrations of inorganic nitrogen in the treatment pond over thisperiod were relatively low, well beneath he limits considered toxic tofish (FIG. 2). The elevated ammonia levels around day 40 were due to arepair of the trickling filter. It should be noted that ammoniaconcentrations above 1-2 mg liter⁻¹ NII₄ -N and nitrite concentrationsabove 0.5-1.0 mg liter⁻¹ NO₂ -N are considered to be harmful for fishgrowth. A similar operated control pond, lacking a treatment system, wasoperated for a period of 15 days. During this period concentrations ofammonia and nitrite reached toxic levels (FIG. 3), causing fishmortality from day 10 onwards. From additional control runs (not shown)with different daily quantities of clean water supply, it could beconcluded that at least 250 m³ of clean water was needed daily (fivepond volume changes) in order to reach the low concentrations ofinorganic nitrogen found in the treatment pond.

Organic matter removal

During the experimental period the net removal of organic matter bybreakdown of organic matter in the settling tank and in the fluidizedbed reactor was estimated to be 20-30% of total amount of organic matterentering the fishpond daily. It was found, furthermore, that short-chainfatty acids, in particular acetate, which were released throughdecomposition processes in the settling tank, were used as an energy andcarbon source for the denitrifying bacteria in the fluidized bed column.It should be noted that the primary goal was aimed at designing atreatment system capable of reducing the levels of inorganic nitrogen inthe fishpond. From preliminary studies it can be concluded that higherremoval rates of organic matter can be obtained upon changing theretention time in the different components of the treatment system.

I claim:
 1. A closed system for water quality control in intensive fishculture systems, for preventing accumulation of inorganic nitrogen andorganic matter, comprising in combination a fish pond of sweet of saltwater, means for passing water to be treated from said fish pond to asettling tank where organic matter is reduced to CO₂ and mainlyshort-chain volatile fatty acids, a fluidized bed reactor into whichwater from said settling tank is pumped and where nitrate is reduced togaseous N₂ and the short chain volatile fatty acids are oxidized to CO₂by a plurality of anaerobic bacteria in an anaerobic stage, while waterfrom the fish pond is pumped through a trickling filter in an aerobicstage where ammonia is oxidized by nitrifying bacteria to nitrate, andreturned to the fish pond.
 2. A system according to claim 1, here wateris added periodically to the pond to compensate for evaporated water. 3.A system according to claim 1 or 2, where the volume of the settlingtank is from about 3 percent to about 10 percent the volume of the fishpond, and where the retention time is of the order of a few hours.
 4. Asystem according to any of claims 1 to 3, where the trickling filter isprovided with a large internal surface area which supports nitrifyingbacteria, and through which water is passed from about 20 to 40 percentof the volume of the pond per hour.
 5. A system according to claim 4,where for a pond of 50 m³ the trickling filter has an internal surfaceof from about 200 m² to about 500 m².
 6. A system according to any ofclaims 1 to 5, where means are provided for introducing the water fromthe settling tank at a rate of from about 0.5 to about 2 m³ per hour fora 50 m³ fish pond, with a retention time of the order of from about 5 toabout 15 minutes in said fluidized bed.
 7. A system according to any ofclaims 1 to 6, where the lower part of the fluidized bed is filled withan inert particulate material.
 8. A system according to claims 1 to 7,where the supernatant from the settling tank, rich in organic material,and especially in short-chain fatty acids is fed to the fluidized bedreactor, where it serves as carbon source for the denitrifying bacteria.